Coloring materials



. Patented Eebz' 15, 1938 UNITED STATES PATENT OFFICE COLORING MATERIALS Jersey No Drawing. Application June 19, 1935 Serial No. 27,418

4 Claims. (Cl. 134-41) This invention relates to photographic reproduction in color and more particularly to improvements in color reproduction by a photomechanical process whereby more accurate results can be ob- 5 tained than is possible with the processes at present known.

This application is a continuation in part of my pending application Serial No. 688,828 filed September 9, 1933 for Photomechanical color reproduction, which issued July 16, 1935 as Patent In photomechanical process work the sketch to be reproduced is photographed through appropriate filters upon'a series of sensitive plates to provide a like number of color separation images from which the printing plates are prepared in any well known manner.

It is well known that the visual or subjective brightness of standard palette colors differs sub- 29 stantially from their actinic or photographic brightness and that the color separation images obtained from a sketch made withthese colors will not produce in the final print correct color rendition even with the best contemporary print- 25 ing inks. In practice, this deficiency is generally 1 combatted by retouching the photographs and working the plates, which practice requires highly "skilled artisans and is both costly and time con- It has been proposed to modify the colors of the original sketch by applying dyes on the sur- 1" face or on a superimposed film, so as to falsify their visual values and/or reflecting powers in such a manner as to compensate for the deficien- 35 cies in the process as it has been realized in practice, which are due to the limitations inherent in the. manner of printing, and in the available pigments that must be used for making the original and for compounding the printing inks. However 40 this is merely a method of interposing hand retouching at a different point'in the reproduction process.

Other related methods place a great burden upon the artiitzand the purchaser by distorting 45 the visual ap arance of the sketch so that he must think in terms of one color value and paint inadifferentcolor value. As a consequence oi! my invention, the original sketch is corrected without handicapping the artist or appreciably distorting 50 the visual appearance of the sketch due to the fact that the correcting procedure of my p focess is mainly technical rather than artistic.

It is an object of my invention to provide special coloring materials for creating an original to be 55 reproduced by a photomechanical process.

Other objects and advantages of my invention will appear from the following description and its novel features are pointed out in the appended claims.

The special coloring materials 'of my invention 5 are obtained by incorporating in certain of them luminous or fluorescent material in ratios. such that suflicient actinic light will be available to expose the photographic material the desired amount without over-exposing the other parts of 10 the original. The fluorescent material may be incorporated in any suitable way as by dissolving it in any solvent which is miscible with the vehicle of the paint and then mixing it with the various colored paints ,by rapid stirring, or the fluorescent 15 compound may be mixed dry with a given pigment and the mixture then ground with the vehicle, or the pigment particles may be coated with the fluorescent compound, as from a solution in a volatile solvent, or by precipitation. After removal of the solvent, there remains a dry, fluorescent, pigment powder, that can be used in paints, pastels, crayons. "carbon tissues, etc. Any desired fluorescent material may be used,'namely, fluorescein, B-naphthol-disuifonic acid, sodium salicylate, luminous alkaline-earth sulfides, anthracene, phenanthrene, anthranilic acid and I have found diethyl dihydrocoilidine-dicarboxylate especially satisfactory for this purpose. The quantity of fluorescent material which should be used depends,

among other things, upon the light source, the

panchromatic emulsion, vthe technique and the coloring materials used. In general the quantity will be determined by the additional luminosity needed to effect color correction as above described and ordinarily the red coloring materials will require a smaller quantity than the blue and the yellow will probably require none at all. Again, some pigments have more or less fluorescence of their own. This must be taken into ac- 40 count by adding enough of the fluorescent compound to supplement the natural fluorescence,

and raise it to the required level of intensity.

when some of the organic fluorescent compounds mentioned above are exposed to radiation of vwave length shorter than 500 millimicrons,

their fluorescence is more or less rapidly destroyed. Strips of paper impregnated with them and exposed for 30 minutes at a distance of.2 feet from a 35 ampere, 100 volt white flame arc show marked destruction of the fluorescence.

Anthracence fades more than diethyl dihydrocollidine dicarboxylate, and anthranilic acid more than anthracene. Phenanthrene loses its fluov rescence much more rapidly than any of the three 2 up p.1

just mentioned. The rate of fading is important to the commercial success of the color reproduction process, because a sketch must be painted in daylight or its equivalent, at a rather high level of brightness. While being made, the sketch maybe exposed continuously to this light for from 1 to over 100 hours. It is exposed again by the client and by the reproducing plant staff, and during reproduction it is exposed for some minutes to intense light from sources rich in ultra-violet radiation.

I have discovered that in the cases of certain solid organic compounds, destruction of the fluorescence by irradiation occurs only when a supply of oxygen is available from the environment. Panels impregnated with anthracene and diethyl dihydrocollidine dicarboxylate, surrounded by a non-oxidizing atmosphere, such as illuminating gas, although exposed to a powerful light source for many hours, show no lowering of the fluorescence intensity. Under these conditions, anthranilic acid is improved in stability, but its behavior is erratic. Phenanthrene, likewise, is not fully stabilized, its fluorescence largely disappearing in 4 hours exposure 4 feet from a 35 ampere whiteflame arc. Dipiperonal acetone has a strong yellow fluorescence which fades in a few seconds exposure to the are light. Its rate of fading is not affected, to any marked extent, by exclusion of oxygen. 1

While unprotected anthracene is much less stable than diethyl dihydrocollidine dicarboxylate, when oxygen is excluded both compounds, in the solid state, achieve practically complete stability of fluorescence.

The exclusion of air from the particles of the fluorescent compound may be accomplished by coating them with certain colloids which are themselves resistant to the action of light and air. For example, if anthracene or diethyl dihydrocollidine dicarboxylate, both of which are relatively insoluble in water, is ground in an aqueous solution of 8% gelatin or 25% gum arabic, and the water is evaporated, the coated compounds withstand exposure to light without loss of fluorescence several hundred percent better than the unprotected compounds. Under these conditions, the anthracene becomes more stable than the diethyl dihydrocollidine dicarboxylate.

If the fluorescent compound has been coated upon pigment particles, a subsequent coating with a suitable colloid, such, for instance, as gelatin or gum arabic, stabilizes the fluorescence to a greater or less extent, depending upon the specific pigment and the specific fluorescent compound. I have found, for instance, that when the pigment used is merely a white flller, stabilization of the fluorescence is more readily attained when the filler is one of relatively low oil absorption, as for example blanc fixe or powdered silica, than when a filler of relatively high oil absorption, such'as china clay, magnesium carbonate, or aluminium hydroxide, is used. Diethyl dihydrocollidine dicarboxylate, coated on a filler and protected by gum arabic, fades more than anthracene coated on the same filler and protected in the same way.

For three-color photographic prints, such as are made by modifications of the carbon and imbibition processes, three pigments or dyes are used of the following hues: yellow, bluish-red, and greenish-blue. As giving a more extended explanation of my process, however, it is more enlightening to consider the problem of painting a commercial sketch or picture in full color. For multi-color painting, a set, or palette, of pigments must be provided that will satisfactorily cover the color scale. As an example of a typical minimum palette for such purposes I might list ultramarine blue, (colloidal sulfide), chromium, oxide green, (chromium oxide), Hausa yellow, (Schultz and Lehmann No. 84), cadmium orange, (cadmium sulfide) permanent geranium lake, (General Dyestuffs Corp.), carbon black (carbon), and titanium oxide white, (titanium oxide).

In the reproduction of ultramarine blue by the contemporary 4-color process, a normal color separation gives one practically as much red as blue, and a considerable amount of yellow and black, so that the actual ratios of printing inks printing together to make ultramarine blue would be of the following order: blue 90, red 80, black 50, yellow 15. As craftsmen known, this gives a purplish-black. The current practice is to adjust the various densities by laborious and difficult hand retouching in one form or another until the ratios are roughly, blue -100, red 15-20, black 0-5, and yellow 0. Ultramarine blue has no natural fluorescence, but by compounding fluorescent material with it, it is possible to make supplementary exposures to the fluorescent light and build up the densities of all parts containing that pigment until the required density is reached on the separation negatives. If the exposure were exactly made, and if no improvements were required over the original, all need for retouching would be removed, but, in any case, the major part is easily eliminated.

The chromium oxide green would be rendered in normal 4-color negatives as roughly, blue 50,

, red 30, black 40, yellow 40, which with printing inks would give a dark grey with a mere suggestion of blue-green. It is necessary to reduce the ratios to 0-5 in the case of the red, and 5-8 in the case of the black, which can be done by supplementary exposures if the green is also made fluorescent.

It is convenient to make the brightnesses, or intensities, of the fluorescences in the blue and green proportionate, so that in making the red printing negative, which requires the most correction, the plate can be exposed to the full fluorescence without having to use a filter to modify either the blue or the green.

The permanent geranium taken in this example has a natural red fluorescence. This is augmented so as to maintain the fluorescence of purples made by mixing the red with the blue. Permanent geraniumis a bluish-red.

In the limited palette given in the present example there are no other colors requiring correction. It will be seen that high fluorescence is only required in those pigments requiring process blue ink in their reproduction, especially if the separate-exposure method, described in my above identified application is followed.

It is to be understood that phosphorescent material may be used for the luminous substance instead of fluorescent material in which case the supplementary exposure would be made in the dark and a relatively long exposure would be necessary.

While my invention has been described as applied to photomechanical process work it is obvious that it may be used wherever color separation images are to be made from subjects which are originally prepared for this purpose, or which are of a nature such that they may easily be altered as required by my invention. Such alteration may readily take the form of retouch= ing the subject with a transparent or translucent fluorescent solution.

In the claims, the term color saturation is to be read as meaning the percent reflection or transmission of the characteristic hue. Thus low saturation colors denotes colors having relatively low percents of reflection or transmission of their characteristic hues.

What I claim as new, and desire to secure by Letters Patent of the United States is:

1. A palette for preparing camera copy in a photo-mechanical process comprising a series of coloring materials differing in reflecting power, certain of the materials having incorporated therein a quantity of fluorescent material, the material possessing the lowest reflecting power containing the largest quantity of fluorescent material.

2. A set of artists colors having incorporated rial approximately inversely proportional to the reflecting power thereof, the particles of luminescent material being covered by a colloid not deleteriously affected by light and air.

ALEXANDER MURRAY. 

